https://doi.org/10.1016/j.seppur.2020.118193
“The search for plausible solvents for CO2 absorption covers a wide range of investigations, from the typical ones with benchmark aqueous amines [1] to curious alternatives such as human fat [2]. Along these investigations came water-lean solvents, better known in previous years as hybrid solvents. Somewhere along the line, researchers realized that mixing organic diluents to aqueous amines could enhance the physical absorption capabilities of the solvent without affecting too much their chemical properties. The name ‘hybrid solvent’ reflects this combinatorial effect.
This emphasis on combination, on aiming for the best of both worlds, has visibly shifted as time went by. Nowadays, many of the most promising water-lean solvents, such as the CO2BOLs [3], [4] or those developed by Barzagli et al. [5], [6], rely on alternative mechanisms of reaction other than the ones observed in typical aqueous amines. Together with this shift came the gradual changing of nomenclature. What these solvents have in common is a reduction in water content and substitution by an organic diluent. Therefore, the name ‘water-lean solvents’ became certainly more descriptive. However, it is also a bit vague. Now that the pretext of physical + chemical absorption seems to be somewhat obscured, what exactly do water-lean solvents have in common? And what do they aim to achieve?”
“Water-lean solvents, or rather hybrid solvents, have experienced a history of shifting interests loosely guided by two distinct factors. The first of these factors, in a pretty straightforward fashion, is the conditions of the raw gas that one wants to treat and the quality of the product one wants to generate. The second, a bit more abstract, is what one perceives as being the ideal properties of a solvent for CO2 absorption.
Regarding the first factor, one could consider Fig. 1 adapted from the book Fundamentals of Natural Gas Processing [19]. In Fig. 1, one can identify the locus for hybrid solvents in the context of gas qualities. Hybrid solvents are indicated for absorbing CO2 from gases with high impurity content and, more importantly, high pressures, adding up to CO2 partial pressures above 700 kPa (7 bars).”
“In this conception, hybrid solvents are viewed in light of their double-absorption qualities. The keyword for them is capacity. The performance of these solvents is seen to override that of aqueous amines at high pressures due to the crossing-over factor (which we will discuss in detail in Section 4). This is perhaps too restrictive. The demand of 700 kPa of CO2 partial pressure essentially bars the utilization of hybrid solvents in any post-combustion capture applications, as it does with biogas upgrading applications. This means that hybrid solvents should only be suitable for very particular pre-combustion capture scenarios. In the 1960s and 1970s, industrial processes such as the Amisol® process [20], the Sulfinol® process and the Selefining® process gained territory in pre-combustion capture [21], all operating with hybrid solvents.
One important remark is that it was often not the intention of these treating processes to generate a CO2 stream that was pure enough for CSS (carbon capture and storage, i.e. either geological storage, enhanced oil recovery or selling as a product). Their goal was producing natural gas with high heating value. With the aim shifting in the past few decades towards climate change mitigation technologies, the CO2 purity obtained by most physical solvent treatments starts to become a problem [21]. Standing on the edge between physical and chemical absorption, hybrid solvents developed then might not be as interesting now. Another interesting factor is the higher selectivity that hybrid solvents have for H2S instead of CO2 absorption, a feature often perceived as a positive point of these technologies in the context of natural gas treatment [21]. Many early investigations measure both H2S and CO2 solubilities in hybrid solvents [22], [23], [24]. Isaacs et al. [25], for example, clearly show that the Sulfinol-D® solvent excels in H2S absorption at acid gas partial pressures way below those for the crossing-over with CO2 when compared with aqueous DIPA.
The 1980s and 1990s saw some publications on hybrid solvents highlighting their vapor–liquid equilibria and CO2 capacities. And then, in the new millennium, more researches started to focus on a different aspect: volatility. According to a number of investigations, the potential for low volatilities in water-lean solvents renders them attractive due to the possibility of recovering the loaded amine while incurring in less latent heat expenditures (i.e. less heat is directed towards the vaporization of the solvent). Studies that propose mixing MEA with diluents such as glycerol [26] and coconut oil [27] are very clear in highlighting this aspect of water-lean solvents. In fact, low volatilities have also been a driving force in the popularization of ionic liquids [28], and Shamiri et al. [26] have explicitly declared their intent in developing an inexpensive green ionic liquid-inspired solvent when mixing MEA with glycerol.
Some other researchers are also optimistic about the huge potential for diversity in water-lean solvent formulation. Given this diversity they can, for example, resort to computational techniques to quickly evaluate the best combination of chemicals to obtain just the right mixture for an hypothetical solvent based on a set of key properties [29], [30]. With water-lean solvents, the possibilities could be endless. This clearly shows the shifting of purposes observed for water-lean solvents from the 1970s to the 2010s.
The formation of a secondary phase upon CO2 absorption, either through liquid–liquid separation or solid precipitation, had been usually seen as an undesirable aspect of some water-lean mixtures. As an example, Leites [31] mentions having screened more than 130 possible diluents for solvent formulation, promptly rejecting all which lead to phase separation. Conversely, the arrival of biphasic systems as possible advantageous CO2 absorption techniques [8] has recently lead researchers such as Karlsson et al. [32] to specifically target diluents that brought forth carbamate precipitation. Many recent studies focus solely on phase separation in water-lean solvents.
Shifting perspectives have brought a renewal of interest in water-lean solvents. And yet, careful reading of past literature would do well to many of the newcomers. The slow absorption rates in mixtures containing very viscous compounds can be foreseen in the seminal work of Woertz in 1972 [33] and keeps being rediscovered. The benefits of low volatilities in water-lean blends, with the caveat that solvent regeneration should perhaps require stripping with an inert gas, is demonstrated by Rivas and Prausnitz in 1979 [34]. Right now, water-lean solvents are being considered for CCS applications. These applications require that CO2 is produced with a high degree of purity, something that is not convenient if desorption must be performed with a stripping gas. Or perhaps an application for water-lean solvents other than CCS will become attractive again. One is left to wonder if the extremely high CO2 content (more than 40 %v/v) typical of natural gas extracted from recently discovered oil fields such as the Brazilian Pre-Salt [35], [36] could provide a renewal of interest in hybrid solvents.”